1. Field of the Invention
The present invention relates to a sector antenna apparatus suitable for use in, for example, wide-angle sensor radar systems, etc., and to a vehicle-mounted transmission and reception apparatus, such as a radar apparatus or a communication apparatus, having the sector antenna apparatus.
2. Description of the Related Art
A first sector antenna apparatus of the related art is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 11-168318 in which a plurality of horn antennas which extend radially are switched by an antenna changeover switch so as to emit beams of high-frequency electromagnetic-wave radiation (radio-frequency signal), such as microwaves or millimeter waves, in, for example, all 360° directions.
A second sector antenna apparatus of the related art is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 9-284045 in which a plurality of element antennas with directivity in different horizontal planes are positioned at different heights so that the central axes of the element antennas are substantially aligned in the vertical direction.
In the second sector antenna apparatus of the related art, some of the element antennas have a different horizontal aperture length so as to cover all directions at a crossover level of −3 dB in adjacent element antennas and at substantially the same gain.
In the first sector antenna apparatus of the related art, the shape of the horn antennas is generally designed so that the horn antennas have the same antenna characteristics, such as beam width and antenna gain. Therefore, the sector antenna apparatus has substantially the same directivity in all directions.
However, a sector antenna apparatus applied to, for example, a vehicle-mounted wide-angle sensor radar apparatus must be able to detect objects, such as obstructions and other vehicles, distant from the vehicle in the forward direction, whereas such a sector antenna apparatus can only detect objects, such as obstructions, near the vehicle in the right and left direction. Therefore, different antenna characteristics may be required depending upon the detection direction.
A sector antenna apparatus having the capability of detecting objects distant therefrom in all directions requires a high angular resolution for distant detection of objects, and requires more horn antennas to support detection in all directions. Thus, the production cost increases, and the size of the overall sector antenna apparatus also increases.
In the related art, patch antennas are mainly used in a vehicle-mounted wide-angle sensor radar system. The patch antennas generally have narrow-bandwidth characteristics, and mass production of patch antennas with necessary antenna characteristics is difficult due to fluctuation of manufacturing imperfections, etc., resulting in low manufacturing yield. Since the directional characteristics of the patch antennas cannot be freely controlled, the patch antennas may not meet the antenna performance required for the radar system.
In the second sector antenna apparatus of the related art having a combination of element antennas with different horizontal aperture lengths, every two element antennas having the same aperture length are paired so that a first antenna pair has a directivity opposite to that of a second antenna pair, and the pairs of element antennas are positioned at different heights in the vertical direction in order to reduce the size of the sector antenna apparatus in the circumferential direction. This cannot achieve different antenna characteristics, such as angular resolution and antenna gain, in the opposite directions, leading to a problem in that the required antenna characteristics are not necessarily achievable in the required direction.
In the second sector antenna apparatus of the related art, the plurality of element antennas are located at different positions in the vertical direction, thus increasing the size of the sector antenna apparatus in the vertical direction. It may therefore be difficult to apply such a sector antenna apparatus to a system whose height is limited.